A polycarbonate-based resin is a polymer excellent in transparency, heat resistance, and impact resistance and is widely used at present as an engineering plastic in the industrial field.
As a method for production of this polycarbonate-based resin, a method in which an aromatic dihydroxy compound such as bisphenol A, and phosgene are allowed to react directly with each other (interfacial polymerization method) is known as a method for producing a high-quality polycarbonate.
As the industrial production method of a polycarbonate by the interfacial condensation polymerization method, a method in which phosgene is bubbled into an alkali aqueous solution of a bisphenol to form a polycarbonate oligomer having a reactive chloroformate group, and a condensation reaction (polymerization reaction) of the polycarbonate oligomer and the bisphenol is further allowed to proceed in the presence of a polymerization catalyst such as a tertiary amine and an alkali aqueous solution simultaneously with or successively after the formation of the polycarbonate oligomer is adopted. From the viewpoints of solubility, handling properties, and the like, methylene chloride is mainly used as the water-insoluble organic solvent on an industrial scale.
The methylene chloride used in the reaction step is generally recovered and then reused for the reaction step (see Patent Document 1). In addition, the waste water after the polymerization reaction, the waste water formed after a washing step, and the waste water formed after a granulation step contain an inorganic material such as sodium chloride and an organic material such as a phenol and a polycarbonate. In order to remove such organic material from the aqueous phase and clean the waste water, the organic material is extracted and removed from the waste water by using an organic solvent, preferably the same water-insoluble organic solvent as that used in the polycarbonate production step, such as methylene chloride. The water-insoluble organic solvent containing the extracted and removed phenols and polymer is reused for the polymerization reaction step (see Patent Document 2).
It should be noted that among polycarbonate resins, a polycarbonate-polyorganosiloxane polymer (hereinafter sometimes referred to as “PC-POS”) has been attracting attention because of its high impact resistance, high chemical resistance, and high flame retardancy, and the polymer has been expected to find utilization in a wide variety of fields such as the field of electrical and electronic equipment and the field of an automobile. As a method for production of PC-POS, a method in which a dihydric phenol compound and phosgene are allowed to react with each other to produce a polycarbonate oligomer, and the polycarbonate oligomer is polymerized with a polyorganosiloxane in the presence of methylene chloride, an alkaline compound aqueous solution, a dihydric phenol compound, and a polymerization catalyst is known (see Patent Document 3).